Skip to main content

Full text of "Text Book Of Mechanical Engineering"

530                  Centrifugal Tension in Belts.

Example 53.—A leather belt is to transmit 2 H.P. from a pulley
12" diameter on a shaft making 160 revs, per m. Find (i) the
tensions, when the belt embraces half the pulley rim, and yu = -3 : (2)
the belt width when the leather is £" thick.

(1)  Log. Is = -4343 x -3 x 22 = -40905       .'. I: = 1*.

/n                                 7               .                IK         I

TT ^      (Tn-4)27rRN        , _     ^        2x33000x7             .,

H.P. = ^-2---------------andTn~4 =         -          '    — 131 Ibs.

33000                               2 x 22 x-5x160       °

There are two values of Tn, viz., (44- 131) and (2-5 4)

.-. 2*5 4 = 4 4- 131 ;      4 = 87-3 Ibs.       and Tn — 218*3 Ibs.

(2)  w" x -25 x 320= Tn = 218*3       /. w" — 2.$" (Seep, mo.)

Tension in Belt due to Centrifugal Force may be
examined similarly to the fly wheel at Fig 353. The weight of a
cubic inch of leather (w) is '0358 Ibs., and the stress per square
inch becomes

I 2 U)1)   11

--------Ibs.

.-. Total tension on tight side = Tn 4-  ^w* *>"

which is the total force the belt must resist at high speeds.

Creep, Slip, and Speed.—As the belt tension changes
from Tn to /n, a small retrograde movement or creep occurs
due to release of tension, causing the follower to revolve at a
slightly decreased rate. The result is known as slip, and repre-
sents a loss t>f about 2 per cent. The speed of belting should not
exceed 3000 to 4000 feet per m.

Length of Belt (Figs. 532 and 533).—The length between
centres c should not be less than 6 times D if much power is
transmitted, though much less is used with light pressures. It
may be as much as 30 feet. Horizontal belts give better
results than vertical ones, and some inclination should always be
given upright belts if possible. Taking the oj>en belt, Fig. 532,



and Total length of belt = 2/t + /2 + /s